In recent years, with the rapid spread of IT-related devices and communication devices such as a personal computer, a video camera and a cellular phone, emphasis is placed on the development of batteries used as the power source for such devices. In the automobile industry, the development of batteries with high output and high capacity for electric vehicles and hybrid vehicles has been promoted. Among various kinds of batteries, a lithium battery attracts attention due to its high energy density and output.
A lithium battery generally has a positive electrode comprising a positive electrode active material layer, a negative electrode comprising a negative electrode active material layer, and an electrolyte present between the electrodes. In addition, it has a positive electrode collector, which collects current from the positive electrode active material layer, and a negative electrode collector, which collects current from the negative electrode active material layer, as needed.
A lithium battery which uses a combustible organic electrolytic solution as the electrolyte provided between the positive electrode active material layer and the negative electrode active material layer requires safety measures against leakage, short circuits, overcharging, etc. Especially, batteries with high output and high capacity are required to achieve further improvement in safety. Therefore, research and development of all-solid-state batteries have been promoted, such as an all-solid-state lithium secondary battery using a solid electrolyte such as a sulfide- or oxide-based solid electrolyte as the electrolyte.
Concerning all-solid-state batteries, particularly all-solid-state lithium batteries, it has been known that by covering the surface of an electrode active material, particularly a positive electrode active material, with other components, deterioration of electrode active materials can be prevented and stability of electrode active materials can be improved.
For example, Patent Literature 1 discloses an all-solid-state lithium battery using a lithium ion conductive solid electrolyte mainly comprising a sulfide, wherein the surface of a positive electrode active material is covered with a lithium ion conductive oxide. In Patent Literature 1, the surface of the positive electrode active material is covered by the lithium ion conductive oxide in such a manner that: a positive electrode active material powder was charged in a falling fluidized coating machine to form a fluidized bed of the powder, and an alkoxide solution containing lithium and titanium was sprayed, followed by hydrolysis of an alkoxide using moisture in the air and heating.